Thresholds of serum calcium oxalate supersaturation in relation to renal function in patients with or without primary hyperoxaluria

Abstract

Systemic oxalosis is a constant feature in patients with primary hyperoxaluria type 1 (PH1) and chronic renal failure (CRF) and is not prevented by regular dialysis (RDT), because removal cannot keep up with retention and overproduction of oxalate. These patients are candidates to kidney and/or liver transplantation, which should be ideally planned prior to the development of oxalosis. However, methods to detect the presence and extent of oxalosis are invasive and poorly reproducible, and only indirect approaches are feasible. Because supersaturation of body fluids is an essential condition for oxalotic deposits to form, we have assessed serum calcium oxalate saturation (beta CaOx) in 12 patients with PH1 and 26 with PH1-unrelated renal diseases and varying degrees of CRF. Nineteen healthy individuals were taken as controls. beta CaOx was closely dependent on oxalate serum levels. Serum oxalate and beta CaOx were increased in patients with CRF as compared to controls, and were inversely related to GFR, assessed as creatinine clearance. However, at any level of GFR, both were always greater in PH1 patients. From the slopes of the regression of beta CaOx over ClCr, saturation was predicted to be obtained at ClCr ranging 24-34 and 8-11 ml/min/1.73 m2 in PH1 and non-PH1 patients respectively. Based on the dependence of beta CaOx on oxalate, saturation was associated with serum oxalate between 44 and 46 mumol/l, irrespective of either the prevailing GFR or the underlying disease. These simple procedures represent a valuable non-invasive tool to define the risk of systemic oxalosis and may assist in timing of transplantation.